Rectifier



Patented July 4, 1950 UNITED STATES PATENT OFFICE No Drawing. Application July 1, 1948, Serial No. 36,438

12 Claims. (Cl. 175-366) This invention relates to blocking layer devices such as selenium rectifiers and light sensitive devices, and more specifically to means for improving the characteristics of selenium employed in the manufacture of the blocking layer devices.

In the production of blocking layer devices having a semi-conductor layer, for example, selenium rectifiers, a rigid carrier electrode or supporting electrode is provided with a thin coating or layer of selenium. The supporting electrode may be formed of nickel, nickel-plated iron, aluminum, magnesium, beryllium or other metals and alloys. A common practice in the production of selenium rectifiers includes grit blasting the iron disc or plate and plating the disc with nickel. The purpose of the grit blasting is to present a roughened surface to the selenium and to thereby improve the adherence of the selenium to the carrier electrode. The disc is then thoroughly cleaned and a thin film of selenium is applied over the nickel layer.

The selenium film or layer may be formed by a variety of methods. The plate or disc may be heated to a temperature above the melting point of selenium, for example, to a temperature of from 230 C., and the selenium in stick form may be rubbed across the heated plate in order to melt the selenium and form the desired film. Another method of application includes placing a measured quantity of powdered selenium or selenium in pellet form on' the heated plate and flowing the melted selenium over the surface. The melted selenium is usually spread over the heated plate by mechanical means, as with a glass rod. The selenium may also be deposited on the carrier electrode from a vapor phase. The vaporization method is commonly employed in depositing the selenium film upon the light metal carrier electrodes.

The selenium is then transformed into its gray, crystalline state by heat treatment. The coated discs are generally stacked with mica, aluminum or other inert, smooth-surfaced discs or washers in contact with the selenium and between adjacent plates, and the stack subjected to a moderate pressure. The stacks under pressure are then subjected to a relatively low temperature, that is, to a temperature below 150 C., and maintained at this temperature for from one hour to four or five hours. During this stage of the heat treatment the selenium softens so that the pressure produces a layer of selenium of relatively uniform thickness and having a smooth surface.

The stacks are withdrawn from the oven or heat treating furnace and the plates are removed from the stack. The plates are then given a further heat treatment at a temperature approaching the melting point of selenium. This heat treatment is generally conducted at temperatures between about 200 C. and the melting point of selenium, for example, about 210 0., for a period of from fifteen minutes to several hours. During the combined heat treatment, a layer of more or less uniform thickness of selenium is produced having a smooth surface and the selenium is transformed from its amorphous, non-conducting form into its gray, crystalline, conducting form.

The smooth surface of the selenium film is then treated to form a blocking layer and a counterelectrode consisting of a relatively low melting point alloy is applied, as by spraying, over the selenium coating.

The final step in the manufacture of the rectifler plates consists of an electrical forming treatment. This treatment consists of subjecting the plate to either an alternating or direct current until a high resistance is developed in the reverse direction. This step may consist of applying to the plate in the reverse direction a direct current voltage of below about 15 volts or a pulsating direct current voltage starting with a low voltage of about 8 volts and gradually increasing the voltage to about 21 volts over a period of several hours.

In the use of an alternating current, it is necessary to include in the circuit a current limiting resistor because one-half cycle flows in the forward direction of the rectifier plate. The voltage may be about 20 volts and the period required for forming is generally greater than that required when a direct current is employed because only one-half cycle of the current flows in the reverse direction.

Light sensitive devices of the blocking layer class are produced in a similar manner. A supporting electrode is provided with a thin film or layer of selenium in its gray, crystalline modification and an artificial blocking layer formed on the surface of the selenium. A second electrode is then applied over the treated surface of the selenium. This electrode may consist of a lightpermeable film of metal or a metal grid. The application of the light-permeable metal may be accomplished by any of the methods known to the art.

The present invention is directed to means for improving the conductivity of the selenium layer in the forward direction. The invention is not limited to any specific form, type or composition of carrier electrode or counter-electrode. Any desired method may be employed in forming an artificial blocking layer on the selenium surface. In the preparation of selenium rectifier plates any means for electrically forming the rectifier plates may be employed.

The principal object of this invention is to improve the forward conductivity of the selenium layer in blocking layer devices.

A further object of this invention is to increase the ratio of the resistance in the reverse direction 3 as compared to the resistance in the forward direction.

Other objects and advantages of this invention will become apparent from .the following description and claims.

The present invention contemplates incorporating in the selenium film or layer a small amount of a modifying agent selected from the group consisting of cesium bromide and iron cesium bromide, the latter having a formula FeCsBrz, probably a double salt corresponding to the formula FeBrz'-CsBr.

The specific details of v the manufacturing process are dependent upon the type of unit being made and the class of service for which it is desired. For purposes of illustration the production of selenium rectifier plates is described.

The modifying agent is mixed with selenium in any desired manner. Both materialsmay be in finely powdered form and may be thoroughly mixed mechanically as by ball milling. The materials may also be mixed by heating the selenium to a temperature at which it is molten and fairly fluid, for example, about 300 C. to

400 C., and the powdered modifying agent added. The mixture is then stirred or agitated vigorously to distribute the modifying agent in the selenium. The amount of modifying agent added may vary from about 0.005% to about 0.5% but is preferably of the order of about 0.1% of the weight of the selenium. The material is then cooled and comminuted, or if desired may be cast into stick form.

The carrier electrode, for example, a nickelplated iron disc is provided with a layer of film of the modified selenium in its gray, crystalline form. This layer may be formed by heating the carrier electrode and applying the modified selenium in powdered form or by rubbing a stick of modified selenium over the heated carrier electrode. The resulting film or layer is then transformed into its gray, crystalline form by the usual heat treatment. The selenium surface is then treated to form an artificial blocking layer by any of the known methods. The counter-electrode is then applied by spraying a low melting point alloy over the treated selenium surface. The rectifier plate is completed by subjecting the composite unit to an electrical forming process.

The addition of these small amounts of the modifying agents to the selenium decreases appreciably the forward resistance of the selenium and decreases the ratio between the current fiow in the reverse direction and the current flow in the forward direction upon the application of a direct currentvoltage to the plate in the reverse and forward directions. An ideal or theoretically perfect rectifier plate would have no current flow in the reverse direction and the rectification ratio would be zero.

In order to compare the rectification ratio of rectifier plates formed with selenium and rectifier plates formed with selenium containing small amounts of cesium bromide, a sample of selenium was divided into two portions. One portion was heated to about 350 c. and powdered cmium bromide added. The mass was stirred to thoroughly distribute the cesium bromide in the molten selenium. The melted mass was cast in stick form. The selenium contained about 0.1% cesium bromide. The other portion of seleni was melted and cast in stick form.

A series of rectifier plates was prepared from each portion of selenium. Nickel-plated iron were heated to about C. and maintained at this temperature for about 2 hours after which the pressure was removed and the plates heated to about 210 C. and maintained at this temperature for about hour.

The plates were subsequently provided with a counter-electrode and then subjected to the same electrical forming operation. The plates formed from untreated selenium were intended as standard or control plates. A rectified direct current voltage of 1 volt was applied to the individual plates of each series in the reverse and in the forward directions and the current flow measured. The leakage current or reverse current flow in the standard or control plates averaged approximately 0.35 milliampere and the forward current fiow in these plates averaged approximately 0.28 ampere. The rectification ratio of these plates was 1:800.

The leakage current or reverse current flow in the plates made in accordance with the present invention averaged approximately 0.35 milliampere. The forward current fiow in these plates averaged approximately 0.71 ampere. The rectification factor of such plates was 122028.

The improvement in the operating characteristics of plates made in accordance with my invention may be further illustrated by reference to another series of rectifier plates prepared as described above. These plates were formed from a diiferent lot of selenium. At an applied rectified direct current voltage of 3 volts the rectification ratio for the standard plates was 1:930 and that for the plates of this invention was 122060. The improvement in the output of the plates made in accordance with my invention may be also indicated by the current flow in the forward direction at an applied voltage of 3 volts. In the case of the standard plates, the current flow in the forward direction averaged approximately 1 ampere whereas that for the plates made in accordance with this invention averaged approximately 3 amperes.

In order to compare the rectification ratio of rectifier plates formed with selenium and rectifier plates formed with selenium containing small amounts of iron cesium bromide, a third sample of selenium was divided into two portions. One portion was heated to about 350 C. and powdered iron cesium bromide added. The mass was stirred to thoroughly distribute the iron cesium bromide in the molten selenium. The melted mass was cast in stick form. The selenium contained about 0.1% iron cesium bromide. The other portion of selenium was melted and cast in stick form.

A series of rectifier plates was prepared from each portion of selenium. Nickel-plated iron discs having an area of about 12 sq. cm. were used as the supporting electrodes and were heated to about 240 C. The selenium and modified selenium coatings were applied by rubbing the respective sticks over the heated plates and spreading the melted material over the surface with a glass rod. The plates were then stacked with mica discs covering the selenium layer and the stacks placed under pressure. The stacks were heated to for about 2 hours after which the pressure was removed and the plates heated to about 210 C. and maintained at this temperature for about A hour.

The plates were subsequently provided with a counter-electrode and then subjected to the same electrical forming operation. The plates formed from untreated selenium were intended as standard or control plates. A rectified direct current voltage of 1 volt was applied to the individual plates of each series in-the reverse and in the forward directions and the current flow measured.

The leakage current or reverse current flow in the standard or control plates averaged approximately 0.35 milliamperes and the forward current flow in these plates averaged approximately 0.42 ampere. The rectification ratio of these plates was 1 :1200.

The leakage current or reverse current flow in the plates made in accordance with the present invention averaged approximately 0.40 milliampere. The forward current fiow in these plates averaged approximately 1.25 amperes. The rectification factor of such plates was 1:3133.

. The rectification ratios of the control plates and the plates made in accordance with this invention were also determined at an applied direct current eraged approximately 2.5 amperes, the rectificai tion ratio being 1 :2083. The leakage current fiow for the plates formed with selenium containing about 0.1 iron cesium bromide averaged approximately 1.7 milliamperes and the forward current flow averaged approximately 6.7 amperes, the rectification ratio being 1:3942.

The improvements in'the operating characteristics of rectifier plates made in accordance with my invention are apparent from the foregoing data. It is obvious that the numerical values of the leakage current and the forward current will difler with different plate areas, with different applied voltages and also will vary with different lots or samples of selenium. The foregoing values, however, are representative of the improvements effected in the operating characteristics realized in the practice of this invention.

This application is a continuation-in-part of my copending applications entitled Rectifier, Serial No. 563,105, filed November 11, 1944, now abandoned, and Rectifier Plate, Serial No. 563,106, filed November 11, 1944, now abandoned. For so much of the subject matter herein disclosed which is also disclosed in said applications, I claim priority of said applications.

I claim: I

l. The method of producing blocking layer devices which comprises adding to selenium from about 0.005% to about 0.5% of a modifying agent selected from the group consisting of cesium bromide and iron cesium bromide, applying a layer of the selenium containing the modifying agent to a supporting electrode, transforming the selenium into its gray, crystalline form, forming a blocking layer on the selenium surface and applying a counter-electrode over the treated selenium surface.

2. The method of producing selenium rectifier plates which comprises adding to selenium a small amount of a modifying agent selected from the group consisting of cesium bromide and iron cesium bromide, providing a supporting electrode with a layer of gray, crystalline selenium containing the modifying agent, applying a counterelectrode over the selenium surface and subjecting the composite unit to an electrical forming operation.

3. The method of producing selenium rectifier plates which comprises adding to selenium a small amount of a modifying agent selected from the group consisting of cesium bromide and iron cesium bromide, applying a layer of the selenium containing the modifying agent to a supporting electrode, transforming the selenium into its gray, crystalline form, forming a blocking layer over the selenium surface, applying a counter-electrode over the treated selenium surface and subjecting the composite unit to an electrical forming operation.

4. The method of producing selenium rectifier plates which comprises adding to selenium a small amount of cesium bromide, providing a supporting electrode with a layer of gray, crystalline selenium containing cesium bromide, applying the counter-electrode over the selenium surface and subjecting the composite unit to an electrical forming operation.

5. The method of producing selenium rectifier plates which comprises adding to selenium a small amount of iron cesium bromide, providing a supporting electrode with a layer of gray, crystalline selenium containing iron cesium bromide, applying the counter-electrode over the selenium surface and subjecting the composite unit to an electrical forming operation.

6. A blocking layer device comprising a carrier electrode, a layer of selenium containing a small amount of a modifying agent selected from thegroup consisting of cesium bromide and iron cesium bromide and a counter-electrode.

7. A blocking layer device comprising a carrier electrode, a counter electrode and an intermediate layer of selenium between the electrodes. the selenium containing from about 0.005% to about 0.5% of a modifying agent selected from the group consisting of cesium bromide and iron cesium bromide.

8. A blocking layer device comprising a carrier electrode, a counter-electrode and an intermediate layer of selenium between the electrodes.

the selenium containing from about 0.005% to about 0.5% cesium bromide.

9. A blocking layer device-comprising a. carrier electrode, a counter-electrode andintermethe electrodes. the selenium containing a small amount of cesium bromide.

12. A selenium rectifier comprising a carrier electrode, a counter-electrode and an intermediate layer of gray. crystalline selenium between the electrodes. the selenium containing a small amount of iron cesium bromide.

STANLEY 8-FRY.

No references cited. 

